1. Field of the Invention
The present invention relates to a temperature sensor and a method of producing the temperature sensor, which is installed in an exhaust gas system, capable of detecting the temperature of the exhaust gas in an exhaust gas pipe emitted from for an internal combustion engine.
2. Description of the Related Art
There is known a temperature sensor installed in the exhaust gas pipe in the exhaust gas system for an internal combustion engine. Such temperature sensor is equipped with a temperature sensitive element. The temperature sensitive element is capable of changing its electrical characteristics according to the magnitude of the ambient temperature. Such a temperature sensor is configured of a temperature sensitive element, a sheath pin with a pair of built-in signal wires connected to electrodes of the temperature sensitive element, and a front cover case placed at the front of the temperature sensitive element. The temperature sensitive element is covered with the cover case.
A filler composed of a filling material is filled between the temperature sensitive element and the cover case in order to fix and support the temperature sensitive element. For example, Japanese patent laid open publication No. JP 2004-317499 has disclosed a temperature sensor having such a configuration. There is a cement material as the filler. The hardening temperature of the cement material of the filler is approximately 150° C. Because the temperature sensitive element in the temperature sensor is fixed to and supported by the cover case using the cement material of the filler, it is possible to prevent the temperature sensitive element in the cover case from the influence of vibration even if the vibration is propagated from the internal combustion engine. This can also prevent the temperature sensor from suffering serious damage caused by strong vibration or objects colliding with the temperature sensitive element and the cover case, which could break the signal wiring. However, when the temperature sensor is placed in the exhaust gas pipe, in which an exhaust gas of a temperature range of 600 to 700° C. flows, in the exhaust gas system for the internal combustion engine of a vehicle, there is a possibility of causing a gap to open between the cover case and the filler by a difference in thermal expansion coefficient between the cover case made of a stainless steel and the filler made of a cement material. FIG. 6 shows such a state. The generation of a gap in the cover case deteriorates the function of the filler, namely, the function of fixing and supporting the temperature sensitive element to the cover case and protecting the temperature sensitive element from the influence of vibration. The vibration further causes breaking of the filler with serious damage in the temperature sensitive element, and also breaking the electrodes of the temperature sensitive element.
By the way, the temperature sensor equipped with a thermistor element has been widely used in various control operations such as a fuel injection control and an exhaust gas purifying control in an internal combustion engine of a vehicle. For example, such a temperature sensor, which is equipped with a thermistor element as a temperature sensitive element, is fixed to and supported by the wall of the exhaust gas pipe so that the temperature sensitive element is projected into the inside of the exhaust gas pipe in which the exhaust gas flows. The temperature sensor detects the temperature of the exhaust gas which flows in the exhaust gas pipe, and outputs a detection signal corresponding to the detected temperature of the exhaust gas. The detection signal is transferred to an outside device such as a vehicle ECU (electric control unit) through a rear side of the temperature sensor which is projected to the outside of the exhaust gas pipe. In general, lead wires of the thermistor element as the temperature sensitive element are electrically connected to metal core wires of sheath pins built into a metal cover of the temperature sensor, and the temperature sensitive element is accommodated in the metal cover. The temperature sensitive element detects the temperature of the exhaust gas flowing in the exhaust gas pipe based on the electrical resistance change of the thermistor element.
In order to accurately control the temperature of the catalyst placed in the exhaust gas pipe, based on the temperature of the exhaust gas flowing in the exhaust gas pipe, the temperature sensor is required to maintain stable characteristics, when in use and to reliably detect the temperature of the exhaust gas flowing in the exhaust gas pipe.
Japanese patent laid open publication No. JP 2004-317499, as one of related art techniques, has disclosed a temperature sensor having an improved responsiveness. It is preferable to decrease the distance measured from the front end part of the metal cover case to the front end part of the temperature sensitive element in the temperature sensor as much as possible. In the temperature sensor, a gap or space formed between the metal cover case and the temperature sensitive element is filled with a filler, in particular, having a filling density of not less than 75% in the gap. The temperature sensor is required to rapidly conduct the heat energy of the exhaust gas, as a detection target, to the temperature sensitive element through the metal cover case.
Also, the influence of vibration on the temperature sensitive element in the temperature sensor must be considered when the temperature sensor is used under severe conditions. There may be a large amount of vibration at high temperature, for example, in an engine exhaust gas system. In particular, breaking of the electrode wires of the temperature sensitive element by vibration caused by an internal combustion engine and the like must be considered, especially, when the electrodes of the temperature sensitive element are made of metal such as platinum (Pt) or Pt based alloy in order to have a high thermal resistance. In order to avoid this, the filler is filled around the thermistor element in the temperature sensor in order to avoid displacement of the thermistor element in the temperature sensor and to increase the vibration resistance function of the temperature sensor.
Recently, better automobile exhaust emissions control has been required and the temperature sensors are also required to have high temperature resistance, for example, to withstand not less than 900° C. because of being placed in the exhaust gas pipe of the exhaust gas system of an internal combustion engine. However, when the ambient temperature of the temperature sensor with the filler, in the metal cover case, in order to increase its vibration resistance, is not less than 750° C., the characteristics of the thermistor element become unstable. In particular, there is a tendency that the temperature sensor becomes unstable when the temperature sensor equipped with built-in thermistor element made of oxide semiconductor has a high filler density. Such a temperature sensor cannot provide a stable output characteristic, namely, cannot output a stable detection signal to the outside devices such as an ECU.